An example of conventional high frequency power supplies is disclosed in Patent document 1. FIG. 18 is a block diagram showing the configuration of the power supply of Patent document 1.
Patent document 1: JP-A-2001-197749.
This high frequency power supply includes an output power setting unit 41 that decides a value of the high frequency power to be supplied to a load, an oscillation unit 43 that changes a level of an oscillation output based on a signal from a control unit 42 to be referred to later, an amplification unit 44 that amplifies the oscillation output by the oscillation unit 43 with a plurality of amplifying elements (not shown), an output power detection unit 45 that detects an output power output by the amplification unit 44, and a control unit 42 that compares the value decided by the output power setting unit 41 and the value detected by the output power detection unit 45, and provides the oscillation unit 43 with error information between those values, thus controlling such that the high frequency power remains constant.
The high frequency power supply also includes a calculation unit 47 that calculates a DC source voltage Vdc to obtain a maximal amplitude within a range where a voltage between terminals on the amplified side of the amplifying element is not saturated, based on a predetermined characteristic graph or a characteristic function with respect to the value decided by the output power setting unit 41, an error amplification unit 50 that amplifies the error between the DC source voltage Vdc calculated by the calculation unit 47 and a DC source voltage Vdc detected by a DC voltage detection unit 52 (referred to later), and outputs the amplified value to a DC source unit 51, the DC source unit 51 that generates the DC source voltage Vdc, and the DC voltage detection unit 52 that detects a magnitude of the DC source voltage Vdc output by the DC source unit 51.
Under such configuration, when the output power setting unit 41 decides the value of the high frequency power, the oscillation unit 43 changes the oscillation output level based on the value of the high frequency power, and the amplification unit 44 amplifies the oscillation output. Meanwhile, the calculation unit 47 calculates the value of the DC source voltage Vdc according to the value of the high frequency power, so that the DC source unit 51 generates the DC source voltage Vdc and supplies the voltage to the amplification unit 44. The amplification unit 44 amplifies the oscillation output based on the DC source voltage Vdc, to supply the amplified oscillation output to the load, for example via an impedance matching device (not shown).
The high frequency power supply may be connected, via the impedance matching device, to a load such as a plasma processing apparatus that processes semiconductor wafers by CVD or etching. In the plasma processing apparatus, the impedance of the load often fluctuates depending on the type and pressure of the gas employed in a plasma chamber, as well as on the value and supply period of the high frequency power to be supplied.
In this case, in the impedance matching device, a variable impedance element (not shown) provided therein changes the reactance (capacitance and inductance), thereby matching the impedance such that the input impedance becomes, for example, 50 Ω.
FIG. 19 is an example of the circuit diagram of the amplification unit 44. The amplification unit 44 is constituted of what is known as a push-pull circuit, and includes a first transformer 53 connected to the oscillation unit 43, a driving voltage supply circuit 54 including a plurality of resistances connected to a secondary winding side of the first transformer 53, a first amplifying element 55 and a second amplifying element 56, for example constituted of a FET (field effect transistor), connected to the driving voltage supply circuit 54, and a second transformer 56 connected to the output terminal of the first and the second amplifying element 55, 56. To the secondary winding of the second transformer 56, the load (not shown) is connected. To the middle point of the primary winding of the second transformer 56, the DC source voltage Vdc generated by the DC source unit 51 is supplied.
FIG. 20 is a waveform chart of the DC source voltage Vdc and a voltage Vp at the output terminal P of the first amplifying element 55 (between the drain and the source of the first amplifying element 55). FIG. 20 indicates that when the amplitude becomes maximal within the range where the voltage Vp at the output terminal P of the first amplifying element 55 is not saturated, the amplification unit 44 incurs a minimal loss. The hatched portions in FIG. 20 represent an extent of the loss, such that the smaller the hatched portion is, the smaller loss is incurred and the higher efficiency is achieved. Here, since FIG. 20 is the waveform chart of the DC source voltage Vdc and the voltage Vp at the output terminal P of the first amplifying element 55, the hatched portions in FIG. 20 does not directly represent the power loss.
In the high frequency power supply, while the load fluctuation is sufficiently mild to allow the impedance matching device to effectively match the impedance, the efficiency is not affected, however when the load fluctuation becomes excessively large or sharp thereby inhibiting the impedance matching, the efficiency is degraded. In the case where a fixed impedance conversion value is employed in the impedance matching device, when the impedance in the load fluctuates downwardly for example, the amplitude of the high frequency voltage supplied to the load is accordingly reduced. In other words, the amplitude of the voltage Vp at the output terminal P of the first amplifying element 55 shown in FIG. 20 is reduced as indicated by dash-dot chain lines, which leads to an increase in loss (area of the hatched portion in FIG. 20), thus leading to degradation in efficiency.
The high frequency power supply shown in FIG. 18 is intended for use under a condition that the load is constant and barely fluctuates, or only fluctuates within such an extent that allows the impedance matching device to effectively execute the impedance matching. The control of the DC source voltage Vdc by this high frequency power supply is, therefore, exclusively executed according to the decided value of the output of the high frequency power to be supplied to the load, without taking into account the degradation in efficiency arising from load fluctuation which disturbs the impedance matching.